The present invention relates to a thermal transfer type color printer with a line thermal head.
Various types of printers are known which melt or sublimate color inks on a surface of a transfer sheet by a thermal head to successively apply the color inks to a recording sheet to print a color image thereon. FIG. 1 shows one of such conventional printers which comprises a clamper 303 for holding a front end portion of a recording sheet 301, a platen roller 302 for winding the recording sheet 301 held by the clamper 303, a pair of rollers 305, 306 for winding front and rear sides of the transfer sheet 304, and a thermal head 308 supported by a shaft 307 to be pressed against the platen roller 302 with a transfer sheet 304 and the recording sheet 301 being interposed therebetween at the time of the printing mode. The transfer sheet 304 has color inks such as yellow, magenta, cyan and black which are successively applied thereon to be striped in the longitudinal directions. The width of each of the ink-applied stripes substantially corresponds to that of one picture plane. Further, the thermal head 308 has a heating section comprising a plurality of heating elements successively arranged in a line in parallel to the shaft of the platen roller 302. In response to supply of a predetermined current signal, each of the plurality of heating elements heat to simultaneously effect a printing by an amount corresponding to one line.
In printing operation, the recording sheet 301 fed from a sheet-feeding tray, not shown, is held on the platen roller 302 through the clamper 303, and then the platen roller 302 is rotated by a motor, not shown, while a current signal is supplied to the thermal head 308 pressed against the platen roller 302 in the state that the transfer sheet 304 is wound around the take-up side roller 305 by a take-up motor, not shown, thereby transferring the color ink of the transfer sheet 304 onto the recording sheet 301. After completion of the first-color printing operation, the platen roller 302 is rotated at a high speed to feed the recording sheet 301 to the recording start position, while transfer sheet 304 is wound around the take-up side roller 305 so that a portion having the next color ink comes into contact with the recording sheet 301 to again perform the similar printing operation. With such a printing operation being repeatedly effected with respect to the same recording sheet 301, the respective color inks on the transfer sheet 304 are overlapped on the surface of the recording sheet 301 to form a color image thereon. After completion of the printing operations by all the color inks, the thermal head 308 is rotated about the shaft 307 clockwise in the illustration to be separated from the platen roller 302 and the recording sheet 301 printed is discharged into a sheet-discharging tray, not shown, thus terminating the printing operation.
However, this FIG. 1 arrangement has a disadvantage that the diameter of the platen roller 302 becomes large so that the size of the thermal head 308 increases, thereby increasing the cost of the printer.
FIG. 2 shows another conventional printer. In FIG. 2, a recording sheet 301 is pressed against a platen roller 302 by pairs of sheet-feeding rollers 309 and 310 without holding the recording sheet 301 with respect to the platen roller 302. For printing, the recording sheet 301 is reciprocated by the sheet feeding rollers 309 and 310.
According to this FIG. 2 printer, although the diameter of the platen roller 302 is small, difficulty is encountered to accurately set the recording sheet 301 at the recording start position in the reciprocative movement of the recording sheet 301, thus deteriorating the image quality due to positional shift.
Further, FIG. 3 illustrates a conventional printer which mainly includes a clamping unit 320 for clamping one end portion of a recording sheet 301, a carrying mechanism 321 for movably supporting the clamping unit 320 in the left and right directions (generally, in the secondary scanning directions of the recording sheet) in the illustration, and a recording unit 322. The clamping unit 320 comprises a clamper 323, a substantially L-shaped pulley block 324 for conveying the clamper 323, and a solenoid 326 fixedly secured to the pulley block 324. Here, the clamper 323 is rotatably fitted to a shaft 325. References 324a and 324b are pullies attached to the pulley block 324. The tip portion of the recording unit 322 side of the clamper 323 is bent so as to appropriately clamp the recording sheet 301 and the other end portion thereof has the solenoid 326 which rotates the clamper 323. Further, the carrying mechanism 321 comprises drive pullies 336, 337, a belt 328 stretched between the drive pullies 336 and 337, and others. The belt 328 is disposed at the lower portion of the clamping unit 320 so as to be driven through belts b and c by means of a motor 327 fixed to the recording unit 322. The pulley block 324 is fixedly secured to the belt 328. Further, below the belt 328 there is disposed a stage 329 for allowing the pullies 324a and 324b fixed to the pulley block 324 to move in the secondary scanning directions of the recording sheet 301. The pullies 324a and 324b are moved by means of an adequate mechanism, not shown. On the stage 329 there is provided a reflection type positioning sensor 330 which is a detecting means to determine the original point (recording start position) of the pulley block 324. The recording unit 322 drives the motor 327 the power of which is transmitted to the platen roller 302 and the sheet-feeding rollers 309 and 310. These sheet-feeding rollers 309 and 310 guide the recording sheet 301 fed. Here, the feeding roller 309 is urged toward the feeding roller 310 side by means of a mechanism, not shown, to slide on the recording sheet 301. Further, on the feeding roller 310 attached to a fixed shaft there is provided a one-way clutch whereby the recording sheet 301 is movable only in one direction. In the motor 327 there is provided an encoder whereby the motor 327 stops to rotate when counting a predetermined number of pulses. The feeding roller 310 is arranged to be rotatable at a lower speed than the drive pulley 337 by changing the gear ratio so as to prevent the recording sheet 301 from being loosen on the travelling passage. The platen roller 302 is disposed between the carrying mechanism 321 and feeding rollers 309, 310 to receive a pressure applied to the thermal head 308. The thermal head 308 is arranged to be rotatable about a shaft 307 and pressed by rotation of an eccentric cam 331 so that pressure is evenly applied to the heating portion of the thermal head 308. Above the thermal head 308 there are provided a pair of transfer sheet rollers 305 and 306 for winding a transfer sheet 304. An intermediate portion of the transfer sheet 304 wound around the transfer sheet rollers 305 and 306 is disposed between the thermal head 308 and the platen roller 302. Numerals 332 and 333 are guide rollers provided at the front and rear sides of the platen roller 302 for causing the transfer sheet 304 to smoothly travel.
According to the FIG. 3 conventional printer, in the reciprocative movement of the recording sheet 301, the recording sheet 301 can accurately take the recording start position to provide a high-quality image and the diameter of the platen roller 302 can be reduced to allow use of a small-sized thermal head 308 to reduce the cost of the printer. However, this arrangement requires the carrying mechanism 321 for allowing the accurate positioning of the recording sheet 301. The provision of the carrying mechanism 321 increases the size and cost of the printer.